Gas plasma vapor etching has been utilized in the past for performing etching operations on semiconductor wafers by exposing the wafers to an active plasma to remove portions of materials carried by the semiconductor structure. The extension of the active plasma etching beyond the patterning of silicon, silicon nitrite and silicon oxide has greatly expanded the horizons of plasma etchings to include, among other things, the etching of aluminum in the production of small geometry integrated circuits. Plasma as compared to chemical etching produces better edge definition, less undercutting, considerably less photoresist adhesion sensitivity, and the elimination of so-called "knee breakdown" due to thinning of the photoresist at sharp edges. This thinning where the aluminum goes over and down the side wall of a cut leads to premature resist failure during wet etching, thus permitting removal of the metal at the near edge.
One of the difficulties experienced in plasma etching has been the failure to achieve uniform etching of the wafer. This can be partially remedied by employing lower etching rates, which cause less effect on the resist, and by using greater spacing between wafers.
It has been further noted that beyond non-uniformity within each wafer, there remains a lack of etching uniformity from wafer to wafer wherein the etching rate decreases as the distance from the center of the circular electrode radially increases. This phenomenon was discussed in applicant's previous U.S. patent application, Ser. No. 928,594, filed July 27, 1978 which disclosed that uniform etching could be accomplished by radially decreasing the spacing between the electrodes wherein the gap between the electrodes is greatest at the center and smallest at the circumference of the electrode.